Culminating with the Kyoto Treaty to reduce greenhouse gas (GHG) emissions in a worldwide effort to reverse the growing effects of global warming, research has been underway to create methods for achieving this goal. It is widely accepted that one of the greatest contributors to global warming is the emission of carbon dioxide (CO2), which is a major product of utility and industrial power plant fuel combustion.
CO2 emissions from power plants around the world, if left unchecked, are expected over the next century to rise from about 370 parts per million today to as high as 1,000 parts per million by the year 2100, as suggested by research conducted by the U.S. Department of Agriculture. Many believe that the only true, sustainable answer to global warming is a dramatic reduction in the overall emissions of CO2, along with a renewable energy strategy that allows carbon to remain sequestered far below the earth's surface in the form of fossil fuels.
The field evolving from these concerns has become known as “carbon sequestration.” To date, the primary research programs, especially those sponsored by the U.S. Department of Energy, have been in the area of burying CO2 in geological formations under ground, under the ocean, or in abandoned mines. All of these strategies are extremely expensive and extremely risky in the event of natural disasters such as earthquakes, tsunamis, and the like.
Contrasting efforts have been made, and there are many patents existing, to create complicated chemical-based processes using catalysts, filters, and cryogenics for positive recovery of pure CO2 for sale into commercial markets. For example, U.S. Pat. No. 5,467,722 to Mercatla uses compression of combustion flue gas and cryogenic separation of pure CO2. U.S. Pat. No. 6,375,716 to Burchell separates CO2 using a complex carbon fiber composite molecular sieve. Further, U.S. Pat. No. 6,447,437 to Lee uses a thermal reactor to convert flue gas constituents to fertilizer. However, the amount of CO2 emitted into the atmosphere dwarfs the market for commercial usage, so these and similar approaches would not make a significant contribution to reducing atmospheric concentrations of CO2 at a reasonable cost.
Further efforts have been made, such as in U.S. Pat. No. 6,619,041 to Marin and U.S. Pat. No. 6,148,602 to Demetri to use oxygen-enriched combustion or gasification to reduce CO2 emissions. Although this approach is valid, the cost to implement is relatively high because of the need for constructing expensive oxygen enrichment facilities.
In contrast to these human-made solutions is nature's own carbon sequestration process called photosynthesis, whereby all vegetation uses the sun's light and takes up CO2 through its leaves to convert CO2 into carbon that is stored in the vegetation's biomass. A great deal of research, largely contributed to by the U.S. Department of Agriculture, is being conducted in the area of photosynthetic sequestration of carbon using open crop lands to absorb higher concentrations of CO2, but this research is aimed at measuring the effects of higher atmospheric concentrations rather than in producing a reduction in the future.
This body of research has also shown that soils can serve as a valuable sink for carbon, but only to the point of achieving an optimum amount of carbon content in the soil, after which the soil cannot absorb much additional carbon. To the extent that the world has vast amounts of infertile soils, creating a strategy with an end product of widespread soil enhancement could have significant benefits in terms of food production, especially in developing countries, while making a major contribution to the reduction of global warming. In addition, if a second end product could be produced in the form of biomass to offset the need for unsequestering the world's buried fossil fuel reserves, a further major contribution could be made.
Several recent patents have addressed the potential for conversion of power plant flue gas to sequester carbon using photosynthesis. For example, U.S. Pat. No. 6,205,704 to Schmitz uses photosynthesis to accept flue gas from the combustion of landfill gas, but does not address major utility and industrial power plant emissions.
U.S. Pat. No. 6,667,171 to Bayless uses microbial cyanobacteria in the presence of solar photons to grow microbes on a plastic membrane inserted into a power plant flue gas stream. A high-pressure water stream is used to harvest the bacteria off of the membrane, but the patent does not address the cost of drying the biomass to provide feedstock for products such as fertilizer, alcohols, or power plant fuel. Because of the large volumes of water used in the harvesting process, this cost could be very high. In addition, the invention is limited to only certain cyanobacteria that can grow in temperatures in the narrow range of 50-70 degrees C. Therefore, a system is needed that will support the growth of a wide range of agricultural species to enable carbon sequestration in many products to maximize the world's potential carbon sequestration.
Long before the concept of global warming was understood, U.S. Pat. No. 3,999,329 to Brais disclosed a logical approach of providing an enclosed atmosphere suitable for plant growth and using the steps of supplying hot flue gas, cooling the flue gas to condense water vapor, keeping the water vapor and flue gas in intimate contact for a period sufficient for the water vapor to absorb at least a portion of the flue gases, and then spraying the condensed water vapor into the growth chamber while passing the cooled flue gas through the same chamber at low velocity.
The Brais patent describes a heat exchanger having a low heat transfer capacity for the purpose of maintaining condensed water vapor and flue gas in intimate contact for as long as possible. The purpose of this strategy was to allow absorption of acid gases and fly ash from the flue gas into the condensate, thus creating an ash-laden slurry for spraying onto plants in the growth chamber, or greenhouse. The Brais patent further specifies air as the cooling fluid for the heat exchanger, which is discharged to atmosphere, and further contemplates a treatment system for the ash-laden condensed water vapor slurry using chemicals prior to spraying on plants in the greenhouse.
One drawback of the Brais system is that the fly ash would likely plug the heat exchanger frequently and make it very difficult to operate reliably with consistent performance. Also, a commercial system of this type would be entirely dependent on proper performance of the heat exchanger, failure of which could adversely affect power plant availability. Other drawbacks include the waste of the heat removed from the flue gas by discharging the air cooling medium to atmosphere, the difficulty in pumping the ash-laden slurry through the system without frequent plugging and severe wear to pumps and valves, the likely damage caused to the plant growth in spraying liquid and ash slurry directly thereon, and the likelihood of aquatic algae growth in the greenhouse that could both rob plants of vital CO2 for accelerated growth rates, but also become toxic to the plants themselves.
Based on this review of related research, it is clear that a solution is needed that can handle large amounts of CO2 from a wide range of utility and industrial combustors and boilers in an enclosed and controlled environment, efficiently produce a wide range of biomass products and a soil amendment product without the need for significant drying, and requiring as little area as possible to allow location of the system immediately adjacent to the power plant, if possible.
Moreover, it would be desirable to provide a system having improved heat transfer capabilities and improved power plant thermal efficiency with minimal waste of heat. It would be further desirable to provide a system with ash collection and treatment means, greenhouse CO2 controls and emissions monitoring.